1. Field of the Invention
The present invention relates to an aberration correcting plate which is mounted to a compatible interchangeable lens for color/monochrome images, used for cameras having different resultant thicknesses of filters (including a glass cover). These filters are provided in front of image pickup devices, and particularly TV cameras, such as CCTV cameras.
2. Description of Related Art
There are two kinds for a camera bodies of CCTV or video camera, etc., for color images and monochrome images. The camera bodies have different kinds of light receiving elements, and different refractive indexes or thicknesses of filters provided in front of the respective light receiving elements, for color images and monochrome images. In a monochrome camera body, a glass cover of the light receiving element whose thickness is approximately 0.8 mm is used as a filter. In a color camera body, there is a low-pass filter in addition to the glass cover. The low-pass filter is usually made of a laminated crystal birefringent plate and an infrared filter and has a thickness of about 5 mm. The refractive index of the filters is approximately 1.5.
The filters are provided on the camera body side, and accordingly, there is inevitably a difference of around 5 mm in total thickness between the filters incorporated in the monochrome camera body and the color camera body.
Consequently, if the interchangeable lens whose aberration has been corrected for color images is mounted to the monochrome camera body, the aberrations, particularly the spherical aberration are considerable, particularly in a lens whose aperture is larger than F1.0, due to the absence of the filter of about 5 mm thickness. To prevent this, it is known to use a medium thickness filter to correct the aberration, to thereby balance the aberration therebetween in a lens having a relatively small aperture. On the other hand, in a lens having an aperture larger than F1.0, a parallel-plate corresponding to the filter for the color camera body is provided at a rear end of the compatible interchangeable lens to correct the aberration.
However, if such a parallel-plate is mounted to the rear end of the compatible interchangeable lens, the parallel-plate interferes with the camera body, depending on the back-focal distance of the lens system or the position of the glass cover on the camera body side, etc., so that it is impossible to mount the compatible interchangeable lens which has been adjusted to meet the color image to the monochrome camera body. If the thickness of the parallel-plate is reduced to prevent the interference, aberration cannot properly be corrected, thus resulting in a deterioration of the image quality.